Stormwater runoff transports varying quantities of pollutants such as oil/grease, phosphorous, nitrogen, bacteria, heavy metals, pesticides, sediments, and other inorganic and organic constituents with the potential to impair surficial water bodies, infiltrate groundwater and impact aquifer systems. The systemic sources of these pollutants are referred to as either ‘point’ or ‘nonpoint’ (sources). Point source pollution is typically associated with a release such as a spill, or “end of pipe” release from a chemical plant. These are considered releases that can be tracked to a single location. Nonpoint source pollution is not readily discernible with respect to a single location, but is associated with combined pollutant loading and deposition from many sources spread out over a large area including a variety of human activities on land (e.g., excess fertilizer runoff), vehicle emissions (e.g., oil, grease, antifreeze), vehicle material wear (e.g., brake pads, metal on metal rubbing, corrosion), as well as natural characteristics of the soil and erosion, climate, and topography. Sediment transport is the most common form of nonpoint source pollution as it can contain a myriad of soluble and insoluble pollutants, comingled and concentrated and easily transported over impervious and pervious surfaces. Nonpoint source pollution via stormwater runoff is considered to be the primary contributing factor in water degradation. Over the past three decades, many studies have been performed to identify the major pollutant constituents typically found in stormwater, and their relative concentrations found in both urban and suburban runoff. Studies have consistently concluded that pollutant levels, particularly in urban runoff, contain concentrations of nutrients and other pollutants, with the potential to significantly impact receiving waters such as streams, lakes, rivers, as well as our underground groundwater aquifer system.
Pollutants in both soluble and insoluble forms such as nitrogen, phosphorous, zinc, copper, petroleum hydrocarbons, and pesticides at various concentrations are commonly found in the stormwater profile. These constituents maintain varying degrees of solubility and transport with some being more mobile than others. Some constituents have a chemical affinity to “sorb” (adsorb/absorb) and collect, or, “hitch a ride,” onto sand particles, sediment, or other non-aqueous matter entrained in the stormwater during transport, thereby increasing the mass of concentration. Sediment laden pollution can also impair waterways due to increased levels of turbidity thereby decreasing sunlight penetration within water bodies, and impairing aquatic life.
Historically, stormwater management systems have relied on collection and conveyance via a network of catchments and underground piping that typically transfer and discharge stormwater to a downgradient water body. Additionally, the practice of stormwater detention and/or retention which relies on the collection or transfer of stormwater to surficial ponds or holding areas whereby infiltration takes place, has been a preferred management technique. Both of these management techniques are commonly referred to as “centralized” techniques which were designed primarily to move stormwater from paved areas, without consideration of the pollutant loading effect.
Beginning in the early 1980's, academia, municipalities, state and federal environmental regulatory agencies began looking at ways to best mitigate problems associated with nonpoint source pollution and stormwater runoff. Instead of relying solely on centralized stormwater collection and conveyance, a more “decentralized” approach to stormwater management began to evolve. Such traditional physical factors in determining stormwater control practices as site topography soil percolation rates, and degree of impervious cover were integrated with strategic land planning in an attempt to best replicate pre-development conditions and preserve the natural process of direct subsurface infiltration of precipitation. The focus turned to ways in which innovative engineering, and systems design and construction practices in new development and redevelopment could best be employed to reduce the impact from increasing the impervious “footprint” thereby minimizing site impact. The term “best management practices” (BMPs) was used to collectively identify various stormwater control practices and methodologies to achieve decentralized versus centralized management by treating water at its source, instead of at the end of the pipe.
Low impact development (LID) is a term used to described a land planning, engineering, and building design approach to managing stormwater runoff. LID emphasizes conservation and use of on-site natural features to protect water quality. This approach implements engineered small-scale hydrologic controls to replicate or mimic the pre-development hydrologic regime of watersheds through infiltrating, filtering, storing, evaporating and detaining runoff close to its source. The LID concept began in Prince George's County, Md. around 1990 by municipal officials as an alternative to traditional centralized control measures. These officials found that traditional practices of detention and retention and associated maintenance were not cost-effective, and many cases, did not meet stormwater management goals, particularly with respect to water quality goals.
Today, LID stormwater management practices have shown in many cases to reduce development costs through the reduction or elimination of conventional storm water conveyance and collection systems and infrastructure. Furthermore, LID systems may reduce need for paving, curb and gutter fixtures, piping, inlet structures, and stormwater ponds by treating water at its source instead of at the end of the pipe. Although up-front costs for LID practices can be higher than traditional controls, developers often recoup these expenditures in the form of enhanced community marketability, and higher lot yields. Developers are not the only parties to benefit from the use of LID atom water management techniques, municipalities also benefit in the long term through reduced maintenance costs.
Of particular interest in regard to the present invention is a BMP practice based on the principals of “bioretention.” Bioretention is typically defined as the filtering of stormwater runoff through a plant/soil complex to capture, remove, and cycle pollutants by a variety of physical, chemical, and biological processes. Bioretention is a practice that relies on gravity to allow stormwater to infiltrate through natural soil or engineered filter “media” complexes while providing some degree of sediment collection/separation, and encouraging microbial degradation of entrained pollutants. Such bioretention practices as “rain gardens” and “sand filters” which rely on infiltration and natural pollutant attenuation began to be incorporated as part of LID practices beginning in the 1990's. In these systems, the ability and rate of water movement is not based upon structural controls, but more a function of the composition of the media and/or soils and the infiltration capacity. Although sand filters provide some degree of bioretention efficacy, more importantly, rain gardens rely on plant systems to further enhance microbial activity, and assimilate and uptake pollutant constituents such as phosphorous, nitrogen, and various metals in their soluble form. Accumulated test data of pollutant removal rates for bioretention practices have consistency shown high levels of control and attenuation. Federal and state environmental protection agencies recognize infiltration practices as the preferred means for returning rainwater runoff to the natural aquifer system, as opposed to piping and discharging collected stormwater to a downgradient water body location such as a river, lake, or the ocean.
Within the past decade, another BMP practice/system which relies on infiltration and bioretention to achieve pollutant removal goals has emerged. This system typically integrates a landscape tree or other plant material with stormwater collection and remediation through an engineered filter media. The system is commonly referred to as a “tree box filter” system. The University of Hampshire Stormwater Center (UNHSC) was one of the earliest institutions to construct and test a tree box filter system. In 2007, UNHSC installed a tree box filter system at their campus test center. The system as designed was an approximately six-foot diameter, three-foot deep, round concrete vault resembling a large inverted concrete pipe. It was filled with a bioretention soil mix composed of approximately 80 percent sand and 20 percent compost. It was underlain horizontally by a perforated “underdrain” pipe at the base of the vault that was connected to, and discharged infiltrated stormwater to an existing stormwater drainage system.
The system also contained an open-topped, vertical bypass pipe near the surface to accommodate heavy stormwater events which would otherwise overwhelm the concrete vault. The vault was open-bottomed to provide some direct infiltration to the underlying soils. The filter media was approximately three feet deep and was designed to maximize permeability while providing organic content by the incorporation of compost and native soils to sustain the tree. The vault was designed to be integrated with a street curb opening to collect surface runoff. During a rain event, stormwater migrating along a street curb would enter the curb cut opening and the vault system. The water then infiltrated through the media and was primarily conveyed through the sub drain pipe to the existing (separate) stormwater drainage system. Although the device had the capability of infiltrating stormwater to the surrounding environment through the open bottom, it principally relied on the sub drain pipe to convey stormwater to the existing drainage system.
Most recently several proprietary tree box filter systems, and other structural bioretention systems, have been introduced for commercial use and are currently marketed as stormwater treatment devices, for the collection, filtration, and discharge of (treated) stormwater. As with the previously described UNHSC system, these systems are primarily vault systems with enclosed walls. They typically are constructed as a water impermeable precast concrete container with four side walls with a perforated horizontal underdrain pipe located at the base of the container. However, in contrast to the aforementioned UNHSC design system, these proprietary systems typically have a water impermeable bottom wall essentially forming a five-sided container, with a partially open top sidewall to allow for plant growth. They are designed to be integrated with street curbside collection with stormwater entering the system via an opening (throat) on one side of the container. The container typically contains a filter media of specific composition, with an overlying organic mulch media layer. The drain pipe collects and conveys filtered stormwater to an outlet point exterior of the container that is typically connected to a downgradient catch basin or other existing stormwater drainage system structure. The drain pipe is typically embedded in a layer of stone to facilitate collection and transport of all infiltrating water to the outlet point. The collection and treatment capacity of these close sided systems are defined by the horizontal and vertical interior dimensions of the container. Plant material is resident in the container with root growth confined within the container. These systems are designed to collect and infiltrate stormwater emanating from aboveground surfaces, underground storm drains, and building roof runoff. Based on third party evaluation and testing data, these systems have proven to provide effective stormwater quality treatment with the capacity to provide substantial pollutant removal rates.
Although tree box filters and other closed box systems have proven to be an effective pollutant removal technology, several perceived deficiencies to their long term efficacy have been identified, which are inspiration and basis of the present invention.
Since tree box filter systems inherently closed systems, both the filter media and plant root systems are contained within a five-sided box, therefore, their identifying name. Not unlike a “pot bound” potted plant, the roots of the plant (particularly trees) within a tree box filter are confined and restricted normally developing and freely migrating beyond the walls of the container.
It is common knowledge that the majority of tree root growth is in a horizontal versus vertical direction. Roots primarily grow and spread laterally outward, and away from the tree trunk in search of nourishment to include water, nutrients and oxygen. Based on documented studies and an accepted understanding of tree root growth by the arboriculture and horticulture community, as well as an evaluation of tree root systems following disturbance or “wind throw”, as much as 80% of a mature tree's root system typically resides in the top 12 inches of soil. Therefore, a tree's root mass exists, and growth takes place, within a shallow horizontal matrix. It is also understood that a tree's roots normally grow to and beyond the distance of its canopy, or outer perimeter of leaf growth, typically by a factor of two or three times the distance between the trunk and outer edge of the canopy. Therefore, a healthy and thriving tree would require an extensive and unobstructed horizontal dimension to develop properly.
A majority of commercial proprietary tree box system containers encompass less than 40 square feet in horizontal dimension. Due to the aforementioned discussion of root growth requirements, an actively growing containerized tree, as typified by a tree box system, would be expected to “outgrow” its horizontal dimension prior to attaining maturity. The negative consequences from the exhaustion of growing area, and the adverse effects of restricting a tree's root system from expanding normally could be the stunting of growth, decline in health, and potential susceptibility to disease and insect infestation. Furthermore, actively growing roots will be deflected in opposing directions following contact with an impenetrable obstacle such as the wall(s) of a tree box container. These roots have the potential to encircle the tree's trunk causing a condition called “girdling” whereby the encircling roots can strangle the tree's trunk as well as other developing roots, choking off nourishment. These debilitating factors could potentially lead to the premature death of the tree. If the tree in a tree box system requires removal and replacement due to decline or premature death, significant labor and material costs would be incurred. To facilitate tree removal, presumably most, if not all of the media within the container would also require removal. This associated cost and labor burden could further be exacerbated due to the potential need to remove existing stone surrounding the aforementioned underdrain piping at the base of the container of the typical tree filter system.
Another perceived deficiency due to the effect of the “consumption” of media space by the ever increasing mass of root growth within the confined space of a tree box system would be the eventual reduction of stormwater movement and infiltration through the media filter. Most commercial tree box filter systems depend on rapid stormwater infiltration through the media to achieve treatment goals. The typical tree box filter media is purposely engineered to be of a highly porous open structure composition, primarily consisting of larger particle gravelly sands, thus providing rapid infiltration, as opposed to common landscape or garden soils that typically contain finer particles of sands, silts, and clay that inhibit rapid infiltration. A lesser percentage of the media mix is typically made up of these latter constituents as well as organic materials such as peat moss or compost that have the ability to absorb and retain water. These constituents are critical in providing irrigation for the tree and to sustain root growth, as well as promoting microbial growth for the degradation of some pollutants. However, it is apparent that the ever expanding network of roots of a maturing tree confined within a tree box would be expected (in time) to interfere with and slow down the infiltration of stormwater, thus reducing operational efficiency of the system.
An additional perceived deficiency with a conventional commercial tree box filter is that since these systems are primarily closed bottomed, the only means to discharge infiltrated stormwater outside of the tree box is by way of the underdrain pipe. Since this pipe is typically connected a downgradient catch basin, or other closed stormwater management system, there is little opportunity to directly infiltrate quantities of this filtered water to surrounding soils and the groundwater system. If the surrounding soils are sufficiently permeable, as previously explained, direct infiltration is the preferred mode for returning rain water, in the form of treated stormwater, to the groundwater system. Therefore, an open bottomed tree filter system could allow quantities of filtered stormwater to be returned to surrounding subsurface soils and ultimately the groundwater system. Additionally, commercial tree box filter systems typically utilize a four or six-inch diameter drain pipe as the sole means to discharge filtered water from the system container. The quantity of water, and speed for which water could be evacuated from the container, are therefore severely limited due to the use of a small diameter outlet pipe as opposed to an open bottomed system such as the present invention.
As previously discussed, tree box filter systems (and other enclosed bioretention based structures) rely on an engineered media of high porosity that allows for the rapid infiltration of stormwater that is entering the system. These medias are composed of inorganic materials to allow for rapid infiltration, and organic materials which retain water within the media to provide irrigation for the plant material. When both inorganic and organic constituents are blended in correct proportions, the resulting engineered media provides a proper balance of high infiltration capacity coupled with sufficient water holding capacity.
Recent studies have determined that the incorporation of specific manufactured products or reconstituted rock-based materials formed by expanding specific minerals under intense heat, often referred to as “ceramics”, into an engineered media that has the capacity to adsorb and absorb (sorption) nutrients commonly found in stormwater runoff. Excessive concentrations of specific nutrients such as nitrogen, phosphorus, and soluble metals are known to pollute soils and water bodies. Sorption occurs as a chemical or physical bonding process where nutrients become “attached” to a material as it passes in aqueous solution. Manufactured products such as activated aluminum and activated iron have shown a great affinity for the sorption of soluble phosphorus and other minerals in the aqueous stage. The incorporation of these materials in an engineered media have shown to provide a measurable reduction in soluble phosphorus in stormwater runoff influent. Ceramics such as expanded shale and expanded clay have also shown a propensity for adsorbing minerals such as phosphorus and nitrogen. The mechanism for this sorption reaction is due mainly in part to the presence of tiny holes and fissures within the lattice of the ceramic structure. These holes and fissures are the result of the artificially induced intense heating of the expanded rock during the manufacturing process that causes the material to “pop”, forming these openings.
Water treatment plant processes employ manufactured products such as coagulants to remove inorganic and organic matter suspended in the untreated source water. Coagulants have the ability to bind small contaminant particles that are suspended in water which otherwise would avoid initial treatment. Water Treatment Residuals (WTRs) are the products produced following this coagulation process, and treatment process. This resulting product may be a thickened liquid or a dewatered solid, in the solid form, these coagulant residual materials may be either aluminum or iron base oxides and are known to have a strong capacity to retain soluble phosphorus. It has been determined that aluminum and iron based WTRs, when exposed to stormwater influent can continue to capture and retain over 90% of soluble phosphorus, even after several years of continued contact.
Incorporating any of these manufactured products including, reconstituted rock, and/or WTRs at no greater than 20% (±5%) by volume with a high infiltrating engineered media achieving an infiltration capacity of greater than 50 (±5) inches per hours would be expected to provide a pollutant removal benefit in systems such as the present invention.
Manufactured tree box filter systems and other enclosed bioretention based structures are currently being used in many parts of the country in both commercial and residential applications where a stormwater management system is essential to mitigate non-point source pollution. These systems are typically manufactured of precast concrete by concrete manufacturers or their affiliates. They are customarily delivered pre-filled with filter media and arrive at a site ready for installation and the incorporation of the final plant product. The primary intent of a closed box system design prefilled with media is to be one of a “packaged” and “drop in place” product, uniform in construction, thereby expediting installation and reducing handling time and associated costs. Essentially closed-bottomed and closed-sided pre-cast concrete water impermeable treatment containers are described in U.S. Pat. Nos. 8,333,885, 6,277,274, 6,569,321, and 8,771,515.
Several advantages to the present invention as to be detailed in the following description are designed to rectify the perceived deficiencies in current tree box filter systems, as well as provide additional benefit. Some of these advantages include, an open sided and open bottomed design to allow for direct infiltration; incorporating an engineered media amended with a manufactured product(s) or reconstituted rock-based materials to provide greater nutrient pollutant removal efficacy; the ability to service street, and building roof runoff; allow for multiple subsurface pipe openings; and, the ability to use a flexible, impermeable or substantially permeable subsurface liner to provide an enclosed treatment area. These, and other advantages will become apparent from a consideration of the following description and accompanying drawings.